1. Field of the Invention
The present invention relates to a magnetic tunnel effect type magnetic head used in a hard disc drive, magnetic tape drive, etc. to read signals from a magnetic recording medium under the magnetic tunnel effect, and a method for producing the same.
2. Description of the Related Art
It is well known as a so-called magnetic tunnel effect that in a laminated structure having a thin insulative layer sandwiched between a pair of magnetic layers, when a predetermined voltage is applied between the pair of magnetic layers, the conductance of a so-called tunnel current varies depending upon the relative angle of magnetization between the pair of magnetic layers. That is, the laminated structure having the thin insulative layer sandwiched between the pair of magnetic layers shows a magneto-resistive effect to the tunnel current flowing through the insulative layer.
With the magnetic tunnel effect, it is possible to theoretically calculate the magneto-resistive coefficient or ratio between the pair of magnetic layers owing to the polarizability of the magnetic layers when magnetized, and more specifically, to have a magneto-resistive coefficient or ratio of about 40% in case the pair of magnetic layers is formed from Fe.
Thus, as a magneto-resistive effect element, the magnetic tunnel junction element (will be referred to as “TMR element” hereunder) having a laminated structure having a thin insulative layer sandwiched between a pair of magnetic layers has been attracting the attention in the field of this art. Especially in the field of magnetic heads, attention is focused on a so-called magnetic tunnel effect type magnetic head (will be referred to as “TMR head” hereunder) using the TMR element as a magneto-sensitive element to detect a magnetic signal from a magnetic recording medium.
The above TMR head is a shielded TMR head in which for example a TMR element is sandwiched between a pair of magnetic shielding layers with gap layers laid between them. By permitting the pair of magnetic shielding layers to work as electrodes, the gap between the pair of shielding layers and TMR element can be reduced.
That is, the conventional shielded MR head is composed of a soft magnetic layer which is to be a lower shielding layer, a nonmagnetic nonconductive layer formed on the soft magnetic layer and which is to be a lower gap layer, an MR (magneto-resistive) element formed on the nonmagnetic nonconductive layer and a pair of conductive layers formed at both ends, respectively, of the MR element, a nonmagnetic nonconductive layer formed on the MR element and pair of conductive layers and which is to be an upper gap layer, and a soft magnetic layer formed on the nonmagnetic nonconductive layer and which is to be an upper shielding layer.
In the shielded MR head, as the gap is decreased for a higher recording density, the nonmagnetic nonconductive layer which is to be the gap layer is thinner. More specifically, because of steps formed by the pair of conductive layers disposed on the opposite ends of the MR element, it is difficult to form the upper nonmagnetic nonconductive layer to a uniform thickness over the MR element. In case the distance between the pair of magnetic shielding layers and the MR element, that is, the so-called gap length, is decreased for playing back a signal recorded with a high density in a magnetic recording medium, it is extremely difficult to secure an insulation between the pair of magnetic shielding layers and the MR element.
On the contrary, in the TMR head, the pair of magnetic shielding layers function as electrodes so that the gap layer can be made thin and thus the distance between the pair of magnetic shielding layers and the TMR element can be decreased. Therefore, in the TMR head, the gap can be made narrow to enable a high density of recording to a magnetic recording medium.
In the conventional shielded TMR head, the pair of magnetic shielding layers having the TMR element sandwiched with the gap layers between them serves also as a magnetic shield which prevents a portion, not to be read, of a signal magnetic field from a magnetic recording medium from being led to the TMR element. For this purpose, the pair of magnetic shielding layers is formed from soft magnetic layers having a conductivity such as NiFe, FeAlSi or the like.
For the magnetic shielding function of the pair of magnetic shielding layers in the shielded TMR head, the soft magnetic layers which are to be the pair of magnetic shielding layers is as thick as about several micrometers (μm) while an insulative layer of aluminum oxide for example, sandwiched between the pair of magnetic layers, of a magnetic tunnel junction layer which is to be the TMR element is as thin as about 1 nm.
In such a shielded TMR head, since the soft magnetic layers for the pair of magnetic shielding layers is very thick, the surface of the soft magnetic layer is very rough. For this reason, for production of the shielded TMR head, the soft magnetic layer, after formed, had to be polished by the so-called chemical and mechanical polishing (CMP) so that the surface of the soft magnetic layer on the lower shielding layer, on which the magnetic tunnel junction layer is to be formed, could be smooth.
Generally, however, the surface roughness of the soft magnetic layer, resulted from the chemical and mechanical polishing (CMP), is on the order of 0.3 nm in center line average height Ra. Thus, in the shielded TMR head, since the insulative layer of the magnetic tunnel junction layer which is to be the TMR element is as thin as about 1 nm, when the surface roughness of the soft magnetic layer at the lower shielding layer side is about 0.3 nm in center line average height Ra, the insulative layer of the magnetic tunnel junction layer is ruptured between the pair of magnetic layers and put into contact with each other, resulting in an electric short-circuit as the case may be.
In this case, since the tunnel current flowing via the insulative layer of the TMR element decreases in the TMR head, so the magneto-resistive coefficient of the TMR element also decreases with a result that the playback output will be considerably small.